Method and apparatus for providing circuit switched domain services over a packet switched network

ABSTRACT

A method of determining a domain mode for a mobile station operating in either a circuit-switched domain or a packet-switched domain. A first domain mode is determined, and then a determination is made whether to rove from the first domain mode to a second domain mode. The determination may be made by a Packet Mobile Switching Center (PMSC) when, for example, the mobile station moves between Routing Areas. Alternatively, the mobile station may make the determination based, for example, on pre-provisioned selection criteria. The mobile station derives a Fully Qualified Domain Name (FQDN) based on a cell identifier of a current cell and uses the FQDN in a DNS query to obtain the IP address of the PMSC.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/884,914 filed Jan. 15, 2007, the disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to circuit switched domain services. Moreparticularly, the present invention relates to providing circuitswitched (CS) domain services over a packet switched (PS) network.

When introducing LTE (Long Term Evolution)—the next generation radioaccess in 3GPP—there are only packet bearers available. This means thatthe wide range of services currently offered within the CS domain cannot be used by a terminal using LTE-connectivity, unless the terminaleither shifts to another radio access or is equipped with a secondtransceiver permitting parallel access to the CS-domain present in 2Gand 3G networks. Examples of existing services are:

-   -   Prepaid    -   Regularity equipments for identity signaling    -   Legal interception    -   0800 calls, free calls    -   Announcements    -   Premium number calls    -   Home Zone    -   Sound Logo    -   Call Barrings    -   MultiSIM    -   Standardized supplementary services

The basic assumption when introducing LTE is that the telecommunicationservice will be provided by the IMS (IP Multimedia Subsystem) system.

BRIEF SUMMARY OF THE INVENTION

The present invention generally describes a method of determining adomain mode for a mobile station. In one embodiment, a first domain nodeis determined. A determination is made whether to rove from the firstdomain mode to a second domain mode according to predefined rovingcriteria.

The present invention also generally describes a method of performingPacket Mobile Switching Center discovery for a mobile station. A FullyQualified Domain Name is derived based on a cell identifier of a currentcell. The derived Fully Qualifed Domain Name is used to determine anInternet Protocol Address of a Packet Mobile Switching Center.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following section, the invention will be described with referenceto exemplary embodiments illustrated in the figures, in which:

FIG. 1 illustrates a system for providing circuit switched domainservices over a packet switched network according to one embodiment ofthe present invention;

FIG. 2 illustrates a 24.008 over 3GPP PS network functional architectureaccording to one embodiment of the present invention;

FIG. 3 illustrates a control plane PS network protocol architectureaccording to one embodiment of the present invention;

FIG. 4 illustrates MS architecture for the CS domain control planeaccording to one embodiment of the present invention;

FIG. 5 illustrates a user plane PS network protocol architectureaccording to one embodiment of the present invention;

FIG. 6 illustrates a state diagram for an MS implementing the 24.008over 3GPP PS network functionality according to one embodiment of thepresent invention;

FIG. 7 illustrates a diagram of a roving criteria method according toone embodiment of the present invention;

FIG. 8 illustrates a diagram of a Packet Mobile Switching Centerdiscovery method according to one embodiment of the present invention;

FIG. 9 illustrates a Packet Mobile Switching Center discovery procedureaccording to one embodiment of the present invention;

FIG. 10 illustrates a Keep Alive process according to one embodiment ofthe present invention;

FIG. 11 illustrates a U8-CSR connection establishment procedureaccording to one embodiment of the present invention;

FIG. 12 illustrates a U8-CSR connection release procedure according toone embodiment of the present invention;

FIG. 13 illustrates a ciphering configuration flow according to oneembodiment of the present invention;

FIG. 14 illustrates network initiated Circuit Switched Signalingaccording to one embodiment of the present invention; and

FIG. 15 illustrates Mobile Station initiated Circuit Switched Signalingaccording to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a wide range of applications. Accordingly, the scope of patentedsubject matter should not be limited to any of the specific exemplaryteachings discussed above, but is instead defined by the followingclaims.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation, aswell as additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawing.

For the purposes of the present invention, the following terms anddefinitions apply:

24.008 over 3GPP PS network: A service which allows a terminal to makeuse of the CS call control also over a 3GPP packet switched network.

24.008 over 3GPP PS network Mode: MS mode of operation where the NASlayers communicate through the U8-CSR entity.

24.008 over 3GPP PS network PMSC: The target MSC, or rather PMSC whenhanding over from CS domain to packet domain entering the 24.008 over3GPP PS network Mode.

24.008 over 3GPP PS network supporting PMSC: The PMCS that a MS willrove into when entering the 24.008 over 3GPP PS network Mode.

3GPP PS network: Radio and core part of the 3GPP Packet Switchedservice.

CS Domain: The domain where Circuit Switched services are executed. Thisdomain can be based on IP in the core network, but on the radio accessthe resources are circuit based.

CS Domain mode: MS mode of operation where the CS related NAS layerscommunicate through either the GERAN RR or the UTRAN RRC entities, thusoperating in the normal CS Domain. When GERAN RR is used, the BSS isconnected to the CN using the standardized A-interface. When UTRAN RRCis used, the RNS is connected to the CN using the standardized Iu-CSinterface.

Discovery procedure: The process by which the MS discovers which PMSC toRove into.

Handover: Mobile station engaged in a call moves between 3GPP CS accessnetworks and 24.008 over 3GPP PS networks.

Handover in: mobile station moves from 3GPP CS access network to 24.008over 3GPP PS network

Handover out: mobile station moves from 24.008 over 3GPP PS network to3GPP CS access network

PMSC: Packet MSC, the network function needed to support the 24.008 over3GPP PS network service.

Supporting PMSC: The PMSC the MS Roves into

Rove in: mobile station reselects from 3GPP CS access network to 24.008over 3GPP PS network

Rove out: mobile station reselects from 24.008 over 3GPP PS network to3GPP CS access networks

Roving: action of re-selection between 3GPP CS access and 24.008 over3GPP PS network for a mobile station in idle mode

Seamless: free from noticeable transitions (i.e. no end-user action isrequired; speech interruptions are short; service interruptions areshort; incoming calls are not missed; packet sessions are maintained;services work identically)

For the purposes of the present invention, the following abbreviationsapply:

AMR Adaptive Multi-Rate

AS Access Stratum

BSS Base Station Subsystem

BSSGP Base Station System GPRS Protocol

BSSMAP Base Station System Management Application Part

CC Call Control

CGI Cell Global Identity

CM Connection Management

CN Core Network

CS Circuit Switched

CSR Circuit Switched Resources

CTM Cellular Text Telephone Modem

DNS Domain Name System

DTM Dual Transfer Mode

ETSI European Telecommunications Standards Institute

FQDN Fully Qualified Domain Name

GAD Geographical Area Description

GAN Generic Access Network

GERAN GSM EDGE Radio Access Network

GGSN Gateway GPRS Support Node

GMM/SM GPRS Mobility Management and Session Management

GPRS General Packet Radio Service

GSM Global System for Mobile communications

GSN GPRS Support Node

HLR Home Location Register

HPLMN Home PLMN

HSPA High Speed Packet Access

IETF Internet Engineering Task Force

IMEISV International Mobile station Equipment Identity and SoftwareVersion number

IMS IP Multimedia Subsystem

IMSI International Mobile Subscriber Identity

IP Internet Protocol

ISIM IMS Subscriber Identity Module

LA Location Area

LAI Location Area Identity

LLC Logical Link Control

LTE Long Term Evolution

MAC Medium Access Control

MM Mobility Management

MS Mobile Station

MSC Mobile Switching Center

MTP1 Message Transfer Part layer 1

MTP2 Message Transfer Part layer 2

MTP3 Message Transfer Part layer 3

NAS Non-Access Stratum

PDP Packet Data Protocol

PDU Protocol Data Unit

PLMN Public Land Mobile Network

PMSC Packet MSC

PSAP Public Safety Answering Point

NOTE: A PSAP is an emergency services network element that isresponsible for answering emergency calls.

PSTN Public Switched Telephone Network

QoS Quality of Service

RA Routing Area

RAC Routing Area Code

RAI Routing Area Identity

RAT Radio Access Technology

RLC Radio Link Control

ROHC Robust Header Compression

RRC Radio Resource Control

RTCP Real Time Control Protocol

RTP Real Time Protocol

SCCP Signaling Connection Control Part

SGSN Serving GPRS Support Node

SIM Subscriber Identity Module

SMLC Serving Mobile Location Center

SMS Short Message Service

SNDCP Sub-Network Dependent Convergence Protocol

SS Signaling Subsystem

TFO Tandem Free Operation

TrFO Transcoder Free Operation

TTY Text Telephone or TeletYpewriter

UDP User Datagram Protocol

UMTS Universal Mobile Telecommunication System

VLR Visited Location Register

VPLMN Visited Public Land Mobile Network

FIG. 1 illustrates a system 100 for providing circuit switched domainservices over a packet switched network. Previously, a mobile station110 using a long term evolution radio access 130 needed to accessanother network, e.g. 2G/3G network 120, in order to use MSC based CSdomain services. The present invention allows a mobile station 110 touse the MSC based CS domain services also when on a long term evolution(LTE) network 130, or other packet based accesses is used. Thisinvention description uses Third Generation Partnership Project (3GPP)accesses as example. It should be noted that other access types also isapplicable to the invention, for example 3GPP2 and WiMax accesses. Thismeans that all the control plane 180 and user plane 190 information forthe CS domain, e.g., circuit switched core network CS CN 160, istunneled through the 3GPP PS network 140. Services related to publicswitched telephone network (PSTN) 170 may also be accessed via CS CN160.

Control plane information 180 is tunneled through packet switchednetwork 140 to network controller 194. Network controller 194 may be astand-alone unit or may be a component of mobile switching center server(MSC-S) 196. User plane information 190 is tunneled through packetswitched network 140 to inter working unit (IWU) 192. IWU 192 may be astand-alone unit or may be a component of circuit switch domain mediagateway (CS-MGW) 198. Network controller 194, MSC-S 196, IWU 192, andCS-MGW 198 are all located in a packet mobile switching center (notshown).

All other possible PS traffic 182 would be transported towards packetdata network 150. The figure shows LTE 130 as an example for the 3GPP PSaccess being used. Other possibilities are e.g. Evolved GSM/GPRS,UMTS/HSPA and WiMax. The LTE example of FIG. 1 also shows Access Gateway(AGW) 144. It should be noted that although the present invention refersto 3GPP accesses, those skilled in the art will realize that any packetbased access may be utilized. AGW 144 is the node corresponding to theGGSN in the Evolved GSM/GPRS and UMTS/HSPA cases. In current standards(3GPP 23.401) the AGW consists of a Serving Gateway and a PDN Gateway(Packet Data Network Gateway).

FIG. 2 illustrates one embodiment of a 24.008 over 3GPP PS networkfunctional architecture 200. Mobile station (MS) 205 contains functionsto access the 24.008 over 3GPP packet switched (PS) network 220.

Packet mobile switching center (PMSC) 225 has new functionality ascompared with a mobile switching center (MSC). PMSC 225 is able tohandle the 24.008 over 3GPP PS network service. In one embodiment, PMSC225 handles interface (U8) 217, which is an interface between mobilestation 205 and PMSC 225 over PS network 220. In one embodiment, PMSC225 is able to request certain IP Quality of service class from the PSnetwork. PMSC 225 may be divided into 4 functional components: the MSCserver 240, a network controller 230, e.g., 24.008 over 3GPP networkController (8PSC), an Inter Working Unit (IWU) 235 and a media gateway245, e.g., CS domain Media Gateway (CS-MGW). Network controller 230 andIWU 235 may be stand alone units or may be part of MSC-Server 240 andCS-MGW 245, respectively.

Network controller 194, 230 terminates the control plane part of the U8interface (i.e. the U8c interface). Network controller 230 optionallyauthenticates the user, e.g., MS 205, before any signaling towards theIu-CS interface is attempted. Network controller 230 is also responsiblefor requesting resources from Policy and Charging Rules Function (PCRF)250.

IWU 192, 235 terminates the user plane part of the U8 interface (i.e.the U8u interface) and is responsible for repacking of the media streamfrom Real Time Protocol (RTP) to Nb (Nb is the interface between mediagateways in the 3GPP CS domain) framing in RTP. Optionally, if anAsynchronous Transfer Mode (ATM) based interface on Iu-CS is used, IWU235 will also do media gatewaying between ATM and IP.

In addition to handling control information from network controller 230,MSC server 240 operates to handle control information from a mobilestation via CS-core 215 and CS access 210 and media control information,Mc, from CS-MGW 245 as detailed in related specifications of the 3GPPstandard. In addition to handling user plane information from IWU 235,Circuit Switched domain Media Gateway (CS-MGW) 245 also operates tohandle media information as detailed in related specifications of the3GPP standard.

MS 205 interfaces to network 210 using Um for GERAN and Uu for UMTSTerrestrial Radio Access Network (UTRAN). The term U(x) is used todenote both Um and Uu without excluding even more interfaces.

The A and Iu interfaces are the two standard CS domain interfacesbetween Access Network 210 and Core Network 215. The A-interface is usedbetween GERAN 210 and MSC 240 and the Iu-CS interface can be used eitherbetween GERAN 210 and MSC 240 or between UTRAN 210 and MSC 240.

The Rx interface is the interface used by applications towards PCRF 250.It is not foreseen that this application will have any additionalfunctionality needs on the interface.

Interface 217, i.e., U8-interface, is divided on user plane U8u 180 andcontrol plane U8c 190. Interface 217 carries all the control and userplane information needed to e.g. Rove; and set-up, maintain, and clear24.008 CS calls over a 3GPP PS network, e.g. network 140, 220.U8-interface 217 is between the MS 110, 205 and the PMSC 225. Theinterface supports all CS defined services within 3GPP 24.008, such asmobility, basic call and supplementary services.

The Gi interface is the IP-based interface coming out from the 3GPP PSnetwork. Another name for the Gi interface is SGi and is used in theSAE/LTE 3GPP specifications. On functional level there is no realdifference between Gi and SGi. In one embodiment, the Gi interface (orthe SGi) is implemented using AGW 144 (which can be broken up into S GWand PDN GW according to current standardisation). In another embodimentusing a GPRS PS network, the Gi interface is implemented using GGSN. Inyet another embodiment using WiMAX, architecture the AGW can betranslated into the Access Service Network Gateway and Home Agent. Inthe WiMAX architecture there are no name corresponding to the Gi and SGireference points.

FIG. 3 illustrates a control plane PS network protocol architecture. Oneembodiment of a 24.008 over 3GPP PS network protocol architecture forthe control plane in LTE is shown. In GPRS and UMTS the protocol stackwill look very similar, however some different node entities exists.

The embodiment shown in FIG. 3 illustrates features of the U8c interfacefor the 24.008 over 3GPP PS network service. The 3GPP PS networkprovides the generic IP connectivity between the MS and the PMSC. Inthis embodiment, the PMSC is connected via the SGi/Gi-interface.

UDP provides transport for the U8 Circuit Switched Resources (U8-CSR)217 between MS 110, 205 and PMSC 225. In one embodiment, U8-CSR is basedon GAN (Generic Access Network) tunneling protocols as defined in 3GPPTS 43.318 and 44.318. The U8-CSR protocol manages the IP connection andperforms functionality equivalent to the GSM-RR protocol. Protocols,such as mobility management (MM) and above, e.g., CC/SS/SMS, are carriedtransparently between MS 110, 205 and PMSC 225. The IP address of MS110, 205 is used by PMSC 225 to communicate with the MS 110, 205 for the24.008 over 3GPP PS network service.

FIG. 4 illustrates MS architecture for the CS domain control plane in MS110, 205. A Connection Management (CM) layer comprises call control(CC), signaling subsystem (SS), and short message service (SMS)services. Mobility Management (MM) layer interfaces with CC, SS, and SMSservices of the CM layer using corresponding service access points(SAPs). The Radio Resource SAP (RR-SAP) interface to the UMTS/GSM-MMlayer is preserved identically for GSM, UMTS and 24.008 over 3GPP PSnetwork (and for GAN if present) access. An access mode switch isprovided to switch between UTRAN/GERAN and 24.008 over 3GPP PS networkmodes (and GAN if present). U8-CSR peers with UTRAN-RRC/GSM-RR toprovide coordination for handover. Note: The RLC/MAC/L1 can be the samein some cases, e.g. in the UMTS CS and PS radio interfaces.

FIG. 5 illustrates a user plane PS network protocol architecture. Theembodiment shown in FIG. 6 illustrates features of the U8u interface forthe 24.008 over 3GPP PS network service. The 3GPP PS network providesthe generic connectivity between the MS and the IP network. CS domainuser plane is transported over RTP/UDP between MS and PMSC. Standard3GPP codecs, e.g. AMR, as specified e.g. in 3GPP TS 26.071, aresupported when operating in 24.008 over 3GPP PS network mode. CS-data istransported over RTP/UDP, by defining a new RTP frame format to carrythe TAF (Terminal Adaptation Function)-TRAU (Transcoder and RateAdaptation Unit) (V.110 like) frames over RTP TTY is transported usingCTM over GSM codec over RTP/UDP. Header compression (such as ROHC) forefficient voice transport over the radio access network can optionallybe supported by 3GPP PS access and PS core nodes.

The U8-CSR protocol provides a resource management layer, which isequivalent to the GSM-RR or UTRAN-RRC and provides the followingfunctions: discovery, i.e. selecting the 24.008 over 3GPP PS networksupporting PMSC; setup of IP bearer resources for CS service between theMS and PMSC (8PSC functional entity); handover support betweenUTRAN/GERAN and 24.008 over 3GPP PS network; and functions such aspaging, ciphering configuration, classmark change, etc.

FIG. 6 illustrates an exemplary state diagram for an MS implementing the24.008 over 3GPP PS network mode. The U8-CSR sub-layer in the MS can bein two states U8-CSR-IDLE or U8-CSR-DEDICATED and moves between thesestates as defined below.

The MS enters 24.008 over PS network mode when the MS switches theserving RR entity to U8-CSR and the SAP between the MM and the U8-CSR isactivated. While the MS remains in 24.008 over 3GPP PS network mode itperforms normal and periodic Location Updates and application levelkeep-alive with the 24.008 over 3GPP PS network supporting PMSC.

The MS moves from the U8-CSR-IDLE state to the U8-CSR-DEDICATED statewhen the U8-CSR connection is established and returns to U8-CSR-IDLEstate when the U8-CSR connection is released. Upon U8-CSR connectionrelease an indication that no dedicated resources exist is passed to theupper layers.

The MS may also enter U8-CSR-DEDICATED state in 24.008 over PS networkmode to from CS Domain mode when Handover to 24.008 over 3GPP PS networkis being performed. In the same way, the MS enters CS Domain mode fromU8-CSR-DEDICATED when Handover from 24.008 over 3GPP PS network.

The 24.008 over 3GPP PS network supports security mechanisms atdifferent levels and interfaces. It is assumed that the 3GPP PS accessis secure enough for the service (using the security mechanism specifiedfor the PS service). Optionally IPsec (IPsec is used by IMS) may beutilized to secure the signaling. Authentication will be done on MMlayer, and controlled by the PMSC (normal SIM and ISIM authenticationprocedures).

In addition, it may be desirable to provide security between the AGW andthe PMSC. For example, the AGW resides in the HPLMN and the PMSC residesin the VPLMN in the roaming case and it might be appropriate to providesome low-level security (e.g. IPsec tunnels) for the traffic between theAGW and the PMSC.

FIG. 7 illustrates a diagram of a roving criteria method according toone embodiment. Method 700 starts at step 705 and proceeds to step 710.At step 710, a first domain mode is determined. Once a first mode isdetermined there must be a way for the MS, e.g. MS 110, 205 to determineif it shall be in CS domain mode or if it shall be in the 24.008 over3GPP PS network mode.

At step 715, a determination whether to rove from the first domain modeto a second domain mode is made according to predefined roving criteria.The decision to rove may be made by the following entities: the PMSC,the MS, the Radio Access or the 3GPP PS CN network. The decision to rovemay be made by one entity alone or in combination with one or more ofthe other entities.

In one embodiment, the PMSC determines whether to rove from a firstdomain mode to a second domain mode. The MS will always be GPRS or PSdomain attached. At Routing Area Update (or equivalent other concept forLTE) when the MS is moving between Routing Areas, i.e. not at periodicRA updates, the MS will connect to the supporting PMSC and ask what modeof operation it shall be in using U8-CSR signaling. The PMSC may have adatabase, or some other means to determine what mode the MS should bein.

In one embodiment, the MS determines whether to rove from a first domainmode to a second domain mode. The MS may have pre-provisioned selectioncriteria. E.g. it shall always be in 24.008 over 3GPP PS network modewhen on LTE or HSPA access. Or, the MS contacts the network (PMSC e.g.or a special database) and downloads Roving criteria, such as certaincells, LAs, RAs or TAs or ranges there of. The Roving criteria will beused when the MS determines which mode to enter, and where to rove in.

In one embodiment, the Radio Access determines whether to rove from afirst domain mode to a second domain mode. The Radio Access may indicateto the MS what mode to select, e.g. via parameters in the SystemInformation messages.

In one embodiment, the 3GPP PS CN network determines whether to rovefrom a first domain mode to a second domain mode. The MS receivesresponses from the 3GPP PS CN network while performing e.g. Routing AreaUpdates and PDP Context activations and modifications. These responsescan also be used to indicate if the 3GPP PS network operator wishes theMS to enter 24.008 over 3GPP PS network mode.

Re-selection between CS Domain Mode and 24.008 over 3GPP PS networkmodes will now be described. Re-selection comprises Rove-in (from CSDomain mode to 24.008 over 3GPP PS network mode) and Rove-out (from24.008 over 3GPP PS network mode to CS Domain mode) procedures.

The Rove-in procedure is applicable when it is determined that the MSshall use the 24.008 over 3GPP PS network service. The access mode inthe MS is switched to 24.008 over 3GPP PS network mode. Theprerequisites for the Rove-in are the following:

-   -   The MS is aware of the appropriate supporting PMSC i.e. the MS        has performed the discovery procedure; and;    -   The MS has performed successful registration towards the PMSC.        The registration process is described later in this description.

When all the above prerequisites are filled, the MS may decide toperform Rove-in to the 24.008 over 3GPP PS network mode (i.e. the MSswitches the serving RR entity to U8-CSR and the SAP between the MM andthe U8-CSR is activated). As part of this, the U8-CSR indicates that CSservice is available and passes the relevant NAS-level systeminformation for the CS domain to the NAS-layers (i.e. MM).

After this normal MM operations follow, e.g. if the LAI indicated to theNAS is different from the previous LAI indicated to NAS or if CS servicewas not previously available, then the NAS-layers in the MS will triggerLocation Updating procedures towards the PMSC as following:

-   -   The MS establishes the U8-CSR signaling connection, with the        24.008 over 3GPP PS network supporting PMSC that approved the MS        registration previously.    -   The PMSC (or very likely the 8PSC entity of the PMSC) is aware        of the cell identity of the current PS cell and either uses that        or another cell identity (as returned to the MS as part of        registration) when indicating the current cell identity towards        the MSC-service entity of the PMSC.

While in 24.008 over 3GPP PS network mode, UTRAN RRC and GERAN-RRentities are detached from the RR-SAP in the MS, as a result theentities do not act on any paging request message for the CS domain inUTRAN or GERAN.

If the 24.008 over 3GPP PS network supporting PMSC rejects theregistration, and does not provide redirection to another Serving 24.008over 3GPP PS network supporting PMSC, the MS shall revert to 3GPP CSaccess network, and start a re-attempt timer. When the timer has timedout, a new registration is attempted, starting with the Discoveryprocedure. If the MS can not revert to CS access network the MS is nolonger reachable for CS domain services.

The Rove-out procedure is applicable when MS determines that it can notbe in the 24.008 over PS network mode, or it needs to detach from the3GPP PS network.

The MS detaches U8-CSR from the RR-SAP and re-attaches UTRAN RRC orGERAN-RR to RR-SAP and restores normal CS domain UTRAN RRC or GERAN-RRfunctionality. There after may follow a normal CS domain Location Updateif CS service is available after rove-out.

FIG. 8 illustrates a diagram of a Packet Mobile Switching Centerdiscovery procedure according to one embodiment. Method 800 starts atstep 805 and proceeds to step 810. At step 810, a Fully Qualified DomainName is derived based on a cell identifier of a current cell. At step815, the derived Fully Qualified Domain Name is used to determine anInternet Protocol Address of a Packet Mobile Switching Center.

FIG. 9 illustrates a Packet Mobile Switching Center discovery procedureaccording to one embodiment. When an MS supporting 24.008 over 3GPP PSnetwork attempts to connect to a 24.008 over 3GPP PS network, the MSneeds to identify a PMSC that supports the 24.008 over 3GPP PS networkservice, a so called 24.008 over 3GPP PS network supporting PMSC. ThePMSC will be identified with a Fully Qualified Domain Name (FQDN) oronly an IP address. The MS can derive the FQDN of the PMSC based oninformation received on a DNS lookup using the unique identifier of thecurrent cell (e.g. Cell Global Identity, CGI, for GERAN cells; or LAIand the 28-bit Cell identity, as described in 3GPP TS 25.331, for UTRANcells) of current PS cell. The important parts of this cell identifierare the Private Land Mobile Network Identifier (PLMN-ID) (consisting ofMobile Country Code, MCC, and Mobile Network Code, MNC), Location AreaCode (LAC) and the Cell identity part. The Location Area Identity (LAI)consists of PLMN-ID and LAC and can be used to uniquely point to an MSC(or MSC pool) in the network. In addition, the Routing Area Code (RAC)might also be used in any combination with the above identifiers. The MSwill construct a FQDN (other than the FQDN of the PMSC) based on thecell identifier of the current PS cell.

For the following example, it is assumed that the MS has determined that24.008 over 3GPP PS network mode shall be entered.

1. The MS derives a FQDN based on the cell identifier of the currentcell. This FQDN could for example take the following format. As anexample, the MS has received the cell identifier of the current cell ine.g. the broadcasted system information in the current cell and itconsists of the following information: PLMN-ID is 012-123 (i.e. MCC is012 and MNC is 123), LAC has value 34567, Cell Identity has value of4567. In this case the built FQDN could for example be:

-   -   lac34567.mnc123.mcc012.pub.3gppnetwork.org or    -   ci4567.1ac34567.mnc123.mcc012.pub.3gppnetwork.org        Once the FQDN is derived, the MS performs a DNS query (via the        IP network) to resolve the FQDN to an IP address.

2. The DNS Server returns a response including the IP Address of therelevant 24.008 over 3GPP PS network supporting PMSC.

Still another possibility for building the FQDN is the following twostep approach. The MS will first receive an operator specific stringduring PS Attachment or PDP Context activation/modification procedures.This operator specific string is valid in the current network until theMS receives another operator string. This string could be e.g.“operatorX.com”. The MS uses that string together with the cell identityof the current cell to build an FQDN for the PMSC. For example, usingthe example from above where LAC of the current cell has value 34567,then “lac34567.operatorX.com” would be built and used towards the DNS toretrieve the IP address of the PMSC.

In one embodiment, the MS performs registration to the appropriate PMSC.The Registration to the PMSC procedure serves the following functions:

-   -   Ensures the MS is registered to the appropriate PMSC entity i.e.        with use of the redirection process;    -   MS informs the PMSC that it is now connected through a 3GPP PS        access network and is available at a particular IP address and        UDP port. The PMSC maintains the registration context for the        purposes of e.g. Paging for mobile-terminated calling; and    -   Provides the MS with the NAS system information for the CS        domain. This “NAS System Information for CS” is delivered to the        MS during the Registration to the PMSC process. This enables the        MS to rove-in, and following the Registration procedure trigger        NAS procedures with the core network (such as Location Area        Update, mobile originated calls, mobile terminated calls, etc.).

The registration is performed as described in the following steps:

-   -   The MS is aware of the appropriate supporting PMSC i.e. the MS        has performed the discovery procedure    -   The MS initiates registration to the PMSC and informs the PMSC        about the current PS cell identity e.g. LAI and cell identifier.        The details of this information may vary depending on the 3GPP        PS RAT being used (e.g. there are some differences between cell        identities in GERAN and UTRAN). The MS also informs whether CS        service is available in the current cell and some other relevant        information e.g. the current PS RAT (GPRS/UMTS/HSPA/LTE) and the        known capabilities of the current PS RAT.    -   The PMSC which accepts the registration and provides the MS with        the relevant NAS-level system information for the CS domain        (e.g. MSC Release Indicator, Attach/Detach allowed, T3212        timeout value (used for Periodic Location Updates) and whether        Call re-establishment is allowed). Note that this NAS-level        system information for the CS domain is normally broadcasted in        the RAN to the MS. As the MS may now be operating in a PS-only        3GPP access network, the PMSC provides the needed information to        the MS as part of a successful registration.

Note: The PMSC may also provide the MS with the Rove-in criteria orreject the registration based on the current 3GPP PS RAT or cellidentity (or parts of it) or other information provided by the MS and/orconfigured in the PMSC. E.g. some LAIs of a specific PS RAT are not tobe used for 24.008 over 3GPP PS network mode and the registration isrejected.

-   -   In addition, the PMSC may provide the MS with the Location Area        Indicator (LAI) and Cell Identity to be used as the cell        identity for the 24.008 over 3GPP PS network mode. If this        information is not provided from the PMSC, then the MS uses the        identifiers of the current PS cell as the LAI and CI also for        the 24.008 over 3GPP PS network mode.    -   The PMSC may also redirect the MS to another PMSC based on the        current 3GPP PS RAT or cell identity (or parts of it) or other        information provided by the MS and/or configured in the PMSC.        The PMSC may also rediret the MS to another PMSC for        load-balancing reasons.

FIG. 10 illustrates a Keep Alive procedure according to one embodiment.The Keep Alive process is a mechanism between the peer U8-CSR entitiesto indicate that the MS is still in the 24.008 over 3GPP PS network mode(i.e. either in state U8-CSR-IDLE or in state U8-CSR-DEDICATED) andwishes to maintain the registration state in the PMSC (i.e. stayregistered and attached) for e.g. mobile terminated calls and so paging.Using periodic transmissions of the U8-CSR KEEP ALIVE message the MS inturn determines that the 24.008 over 3GPP PS network supporting PMSC isstill available using the currently established lower layerconnectivity.

1. The MS sends U8-CSR KEEP ALIVE to the 24.008 over 3GPP PS networksupporting PMSC. The MS includes TMSI in the message as theidentification or if no TMSI is available, then IMSI is used.

2. The PMSC acknowledges the U8-CSR KEEP ALIVE message by sending theU8-CSR KEEP ALIVE ACK message to the MS. This acknowledgement is neededas UDP is used as the underlaying transmission mechanism.

The Deregistration from the PMSC procedure allows the MS to explicitlyinform the PMSC that it is leaving the 24.008 over 3GPP PS network mode(e.g. when it is about to enter a network where the normal CS service isavailable and the MS is not supposed to use 24.008 over 3GPP PS networkanymore), by sending a U8-CSR DEREGISTER message to the PMSC, allowingthe PMSC to free resources that it assigned to the MS.

The PMSC may also autonomously release the MS registration context, andsend a U8-CSR DEREGISTER message to the MS. NOTE: At power-down theU8-CSR sublayer of the MS ensures that the MS explicitly detaches fromthe network, where possible, before completing the Deregistration fromthe PMSC procedure.

The MS is authenticated to the 24.008 over 3GPP PS network service usingnormal MM level Authentication that may take place after the Rove-Inprocedure if the normal Location Update procedure is triggered.

3GPP PS network encryption will be used. Security may also be providedbetween the AGW and the PMSC.

The U8-CSR connection is a logical connection between the MS and the24.008 over 3GPP PS network supporting PMSC. It is established when theupper layers in the MS request U8-CSR to enter dedicated mode. When asuccessful response is received from the network, U8-CSR replies to theupper layer that it has entered dedicated mode. The upper layers havethen the possibility to request transmission of NAS messages to thenetwork.

FIG. 11 illustrates a U8-CSR connection establishment procedureaccording to one embodiment. FIG. 11 shows successful establishment ofthe U8-CSR Connection.

1. The MS initiates U8-CSR connection establishment by sending theU8-CSR REQUEST message to the 24.008 over 3GPP PS network supportingPMSC. This message contains the Establishment Cause indicating thereason for U8-CSR connection establishment.

2. Depending on the reason for U8-CSR connection establishment, the PMSCmay request modification of the signaling bearer from the PCRF to beable to cope with signaling that may follow.

3. Possible response from the PCRF.

4. 24.008 over 3GPP PS network supporting PMSC signals the successfulresponse to the MS by sending the U8-CSR REQUEST ACCEPT and the MSenters dedicated mode and the U8-CSR state changes to U8-CSR-DEDICATED.

5. Alternatively, the 24.008 over 3GPP PS network supporting PMSC mayreturn a U8-CSR REQUEST REJECT indicating the reject cause.

FIG. 12 illustrates a U8-CSR connection release procedure according toone embodiment. FIG. 12 shows release of the logical U8-CSR connectionbetween the MS and the 24.008 over 3GPP PS network supporting PMSC.

1. The 24.008 over 3GPP PS network supporting PMSC decides to releasethe signalling and user plane connection resources allocated to the MS(e.g. in Iu interface that could be seen as an Iu Release Command).

2. The 24.008 over 3GPP PS network supporting PMSC commands the MS torelease the signalling plane and user plane resources, using the U8-CSRRELEASE message.

3. The MS confirms resource release to the 24.008 over 3GPP PS networksupporting PMSC using the U8-CSR RELEASE COMPLETE message and the MSenters idle mode and the U8-CSR state in the MS changes to U8-CSR-IDLE.

4. If resources has been requested in the U8-CSR connectionEstablishment and/or during the traffic channel assignment, the PMSCwill ask for a modification of the signaling bearer from the PCRF. ThePMSC may also ask PCRF to release the Secondary PDP context if such wasallocated for the user plane. This may be uncorrelated to step 2 and 3.

5. Response from the PCRF.

6. All the signaling and user plane connection resources allocated tothe MS are released (e.g. in Iu interface that could be seen as an IuRelease Complete).

FIG. 13 illustrates a ciphering configuration flow according to oneembodiment. The message flow for ciphering configuration is shown. Theterm ciphering configuration is normally used for GSM CS operations andthe corresponding term for UMTS CS (and PS) operations is Security ModeCommand.

1. The 24.008 over 3GPP PS network supporting PMSC sends U8-CSRCIPHERING MODE COMMAND to the MS. This message indicates the integrityprotection and encryption settings (i.e., that may be applicable afterhandover/relocation to CS Domain mode). MS stores the information forpossible future use after a handover/relocation to CS Domain mode. Themessage may also indicate whether the MS shall include IMEISV in theU8-CSR CIPHERING MODE COMPLETE message.

2. The MS then sends U8-CSR CIPHERING MODE COMPLETE message to networkand includes the IMEISV, if indicated so in the U8-CSR CIPHERING MODECOMMAND.

CS signaling in U8-CSR-DEDICATED state is shown in FIG. 14 and FIG. 15.FIG. 14 illustrates network initiated Circuit Switched Signalingaccording to one embodiment in the downlink direction i.e. from the PMSCto the MS.

1. The 24.008 over 3GPP PS network supporting PMSC encapsulates aNAS/Layer 3 message within a U8-CSR DL DIRECT TRANSFER message that issent to the MS.

FIG. 15 illustrates Mobile Station initiated Circuit Switched Signalingaccording to one embodiment in the uplink direction i.e. from the MS toPMSC.

1. For MS initiated signaling the MS U8-CSR layer receives a requestfrom the NAS layer to transfer an uplink NAS signaling message or SMSmessage. The MS U8-CSR encapsulates the message within a U8-CSR ULDIRECT TRANSFER message and sends the message to the 24.008 over 3GPP PSnetwork supporting PMSC.

1. A method of determining a domain mode for a mobile station, whereinthe mobile station accesses a Packet Switched Core Network through aLong Term Evolution, LTE, radio access network, and wherein a PacketMobile Switching Center, PMSC, interfaces the Packet Switched CoreNetwork with a Circuit Switched Core Network, said method comprising thesteps of: determining a first domain mode; and determining whether torove from the first domain mode to a second domain mode according topredefined roving criteria, said determining step including determiningwhether a current serving cell, location area, routing area, or trafficarea requires a change of domain mode; wherein the first domain mode andthe second domain mode comprise a Circuit Switched domain mode and a24.008 over 3GPP Packet Switched network mode.
 2. The method of claim 1,wherein the roving determination is made by the PMSC.
 3. The method ofclaim 2, wherein the roving determination is made when the mobilestation is moving between Routing Areas.
 4. The method of claim 3,wherein the mobile station connects to the PMSC using U8-CSR protocolsignaling.
 5. The method of claim 4, wherein the mobile station sendsperiodic U8-CSR KEEP ALIVE messages to the PMSC while the mobile stationoperates in 24.008 over 3GPP Packet Switched network mode.
 6. The methodof claim 4, wherein the mobile station connects to the PMSC through thesteps of: the mobile station sending a U8-CSR REQUEST message to thePMSC with an establishment cause; the PMSC requesting a modification ofthe signaling bearer from a Policy and Charging Rules Function, ifrequired by the establishment cause; and the PMSC returning anacceptance to the mobile station.
 7. The method of claim 4, furthercomprising the mobile station releasing the connection to the PMSCthrough the steps of: the PMSC deciding to release the signaling anduser plane connection resources allocated to the mobile station; thePMSC sending a U8-CSR RELEASE message to the mobile station commandingthe mobile station to release the signaling and user plane connectionresources; the mobile station sending a U8-CSR RELEASE COMPLETE messageto the PMSC; and the mobile station entering idle mode.
 8. The method ofclaim 7, further comprising the steps of: the PMSC requesting amodification of the signaling bearer from a Policy and Charging RulesFunction; and the PMSC requesting release of a Secondary PDP Context, ifallocated for the user plane.
 9. The method of claim 4, furthercomprising the steps of: the PMSC sending a U8-CSR CIPHERING MODECOMMAND message to the mobile station indicating integrity protectionand encryption settings; and the mobile station storing the integrityprotection and encryption settings for use if the mobile station rovesto the Circuit Switched domain mode.
 10. The method of claim 4, whereinthe mobile station and the PMSC utilize Circuit Switched signaling inU-B CSR-DEDICATED mode, and the PMSC encapsulates a NAS/Layer 3 messagewithin a U8-CSR DL DIRECT TRANSFER message sent to the mobile station.11. The method of claim 4, wherein the mobile station and the PMSCutilize Circuit Switched signaling in U-8 CSR-DEDICATED mode, and themobile station encapsulates a NAS message or SMS message within a U8-CSRUL DIRECT TRANSFER message sent to the PMSC.
 12. The method of claim 1,wherein the roving determination is made by the mobile station.
 13. Themethod of claim 12, wherein the mobile station has pre-provisionedselection criteria.
 14. The method of claim 12, wherein the mobilestation downloads roving criteria.
 15. The method of claim 1, wherein aRadio Access type indicates to the mobile station what mode to select.16. The method of claim 15, wherein the Radio Access type indication isprovided using parameters in System Information messages.
 17. The methodof claim 1, wherein the roving determination is made by the PacketSwitched Core Network.
 18. The method of claim 17, wherein the rovingdetermination is made by a 3GPP Packet Switched Core Network.
 19. Amethod for providing Packet Mobile Switching Center, PMSC, Discovery fora mobile station, said method comprising the steps of: deriving by themobile station, a Fully Qualified Domain Name, FQDN, based on a cellidentifier of a current cell; sending the FQDN in a query to a DomainName System, DNS, server; and receiving from the DNS server, an IPaddress of a PMSC based on the FQDN.
 20. The method of claim 19, whereinan operator specific string received by the mobile station and the cellidentifier of the current cell are used to derive the FQDN.